Emoji Shortcode Support

**Is your feature request related to a problem? Please describe.**
Android and the desktop versions of Element support emoji shortcode. However, iOS is without this feature and causes one to waste a significant amount of time inserting an emoji.

**Describe the solution you'd like**
When I type `:ap`, I'd like to see all available emojis that match that shortcode.
When I type `:apple:` and continue typing, I would like my shortcode to convert to 🍎 .

**Describe alternatives you've considered**
I have to manually search the emojis or copy & paste from another source to get the emoji I want.

**Additional context**
N/A

No files, tests, or entry points are named. Start by locating the iOS composer and emoji input UI, then compare the requested shortcode suggestions and conversion behavior with the Android and desktop implementations. Done means matching shortcodes can be suggested while typing and a completed :apple: shortcode converts to 🍎.

Written by the indexing model from the issue text.